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Fungus

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Fungus


Fungus



The Latin word for mushroom is fungus (plural, fungi). The word fungus has come to stand for a whole group of simple plants that contain no chlorophyll and lack such complex plant structures as roots, stems, leaves, and flowers. Included among the fungi, along with mushrooms, are molds, mildews, rusts, smuts, truffles, and yeasts. Toadstool is another name for mushroom. Some people use the name toadstool only when referring to poisonous mushrooms, but botanists make no such distinction. A general scientific term for fungi is mycota, from the Greek word for mushroom, mykes, and the study of these organisms is called mycology.

Because they lack chlorophyll, fungi are unable to manufacture food out of the raw materials around them as other plants do. They must therefore get nutrition from other plants and from animals. When they get their food from living plants or animals, fungi are called parasites. When they get it from dead plant or animal matter, they are called saprophytes.

Fungi are very widely distributed throughout the world, particularly in the temperate and tropical regions where there is sufficient moisture for them to grow. They are less likely to be found in dry areas. Some few types of fungi have been reported in Arctic and Antarctic areas (some molds, after all, thrive on refrigerated food). There are about 50,000 known species of fungus.

Although any single typical fungus may not be uniform in appearance--a mushroom, for example, has a cap, stem, and rootlike components--it has, in fact, a uniform structure throughout. The typical fungus consists of a mass of tubular, branched filaments, or strands, called hyphae (singular, hypha). The mass of hyphae is called the mycelium, and it is this that makes up the thallus, or body, of the fungus.

In order to grow, the mycelium uses the organic matter, either living or dead, in its environment. As the mycelium matures, it forms spores. These are seedlike reproductive bodies, each normally consisting of one cell, that become detached from the parent fungus and start new organisms. As the spore grows, it develops into a hypha that branches out and eventually forms the mycelium of a new fungus. In some fungi the spores may be produced directly by any portion of the mycelium; in others, such as the mushroom, they are formed in a special fruiting section, such as the mushroom cap. This section, normally the only visible or most visible section of the fungus, is called the sporophore.

Fungi live both on land and in the water. Only a small portion of those that live on land is normally visible. Most of the fungus, a complex network of hyphae, grows underground, near the surface. The visible parts of fungi vary greatly in size. Some are so tiny that they cannot be seen without the aid of magnification. Others are quite large. Some mushrooms reach diameters of 8 to 10 inches (20 to 25 centimeters) and heights of 10 to 12 inches (25 to 30 centimeters). Bracket fungi that are 15 inches (38 centimeters) in diameter are fairly common; and mushrooms called puffballs have been known to grow to 60 inches (152 centimeters) in diameter.

Although fungi are distributed worldwide, the distribution of a specific species is limited by temperature and moisture conditions of an area coupled with the available food supply. The best temperature for most fungi to thrive is from 68' to 86'F (20' to 30'C). Some types of fungi, however, do perfectly well at tem- peratures as high as 120'F (48'C), while a fairly large number of them do well at freezing temperatures, 32'F (0'C) or below.

The reproduction of fungi can be either sexual or asexual. Sexual reproduction, as with other organisms, involves the fusion of two nuclei when two sex cells unite. This joining produces spores that can grow into new organisms. Asexual reproduction is by fragmentation, cell division, or budding. The simplest process is direct fragmentation, or breaking up, of the fungus body, the thallus. Each of the fragments develops into a new individual organism if environmental conditions are favorable. Such fragmentation usually is the result of outside natural forces. Some yeasts, which are single-celled fungi, reproduce by simple cell division. A yeast cell divides into two complete yeast cells. Other fungi, including some yeasts, reproduce by a method that is called budding. A bud develops on the surface of a yeast cell; the nucleus of the cell then divides into two, one of which moves into the bud. The bud is capable of starting a life of its own as a new fungus

How Fungi Obtain Food

A fungus secretes enzymes into the living or dead plant or animal material on which it grows. The enzymes digest the material, which is then absorbed through the walls of the hyphae. A common example of this action is the rotting of fruits such as peaches or apples. The brown, softened area on the fruit has been subjected to the enzyme secretions of the hyphae. Some fungi, more specialized in their food-absorbing techniques, produce tiny hyphae called rhizoids. These rootlike structures anchor the fungus to its food source and probably also absorb food. Other fungi produce absorptive structures called haustoria, another type of hypha outgrowth.

Fungi are classified as parasites or saprophytes (or saprobes), depending on whether they feed on living or dead organic matter. Because parasites attack living organisms for their nutrition, they are a leading cause of disease in plants. Some cause diseases in animals, including humans. Among the fungal diseases that attack plants are downy mildew on grapes, onions, and tobacco; powdery mildew on grapes, apples, cherries, lilacs, peaches, and roses; smut on corn, wheat, and onions; rust on wheat, oats, beans, asparagus, and some flowers; brown rot on some fruits; and various spots, blights, and wilts on leaves.

Because they attack only dead organic matter, saprophytes are in good measure responsible for the decomposition of much plant and animal residue. They also attack foodstuffs such as bread, processed meat and cheese, and picked fruits and vegetables. Some saprophytes are responsible for the destruction of timber, textiles, paper, and leather. Most saprophytes need oxygen in order to survive and feed. Some few, such as those that cause fermentation, can survive without it.

Plant and Animal Associations

Some types of fungi live in close, mutually dependent association with ...

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